1. Field of the Invention
The present invention relates to a mobile communication system and, more particularly, to personal communication services (PCS), digital cellular communication system or similar mobile communication system using a code division multiple access (CDMA) scheme. Also, the present invention relates to a mobile station, a base station and a mobile communication control center (MCC) constituting the mobile communication system. Further, the present invention is concerned with a method of switching a communication channel in order to effect handover between base stations included in the above system, and an arrangement for practicing the same.
2. Description of the Background Art
A mobile communication system, particularly a CDMA mobile communication system controlling transmission power in order to reduce interference between users, is discussed in, e.g., "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System", TIA/EIA/IS-95, July, 1993, U.S.A. (Document 1 hereinafter), Andrew J. Viterbi "CDMA: Principles of Spread Spectrum Communication", Addison-Wesley Wireless Communications Series, pp. 185-195, 1995 (Document 2 hereinafter), and N. Nakajima "Developments on Cellular Configuration Architecture", NTT DoCoMo Technical Journal, Vol. 1, No. 2, pp. 21-29 (Document 3 hereinafter).
Document 1 describes a US standard system relating to a radio interface between a mobile station and a base station included in the CDMA communication system. Document 2 teaches a specific arrangement of base stations in the CDMA communication system described in Document 1. Further, Document 3 teaches the arrangement of base stations in a current digital cellular telephone system called a PDC (Personal Digital Cellular) system, and the sectoring of the base stations.
It is a common practice with the CDMA communication system to define transmission paths between the MCC and the base stations by use of synchronous digital hierarchy (SDH hereinafter), and send information at a transmission rate particular to the SDH. The MCC multiplexes the transmission paths by time division multiplexing and thereby send user information (including speech information and computer data) and control information relating to the mobile stations existing in service areas controlled by the base stations.
In the conventional CDMA communication system, the mobile stations, base stations and MCC each includes a receiver implemented by a global positioning system (GPS hereinafter) and has absolute time. These constituents therefore operate in synchronism with each other. It follows that when a down-going link from the base station to the mobile station is switched from a certain base station in communication to another base station, a plurality of base stations can send the same information in synchronism, allowing the mobile station to perform maximum ratio combination diversity receipt. This frees signals from momentary interruption even at the time of switching of a down-going link. This kind of handover will be referred to as soft handover.
With CDMA communication controlling transmission power for the previously mentioned purpose, it is possible to reduce transmission power by using cell diversity available with soft handover, to increase the number of mobile stations connectable to a single base station, and to thereby enhance the communication efficiency of the entire system.
However, the prerequisite with the conventional technologies is that for soft handover a period of time necessary for information multicast from the MCC at a certain time to reach a base station expected to newly join in communication with a base station be shorter than a period of time necessary for the same information to reach a base station currently communicating with the mobile station. This is because when a speech or similar continuous information is sent, the link between the mobile station and the base station currently in connection must be maintained.
Specifically, the link to be formed from the base station to be connected next and the mobile station must be synchronous with the link currently held as to the transmission of information. Should information fail to reach the base station expected to set up synchronization at the above timing, soft handover would fail. If soft handover is not practicable, the mobile station expected to receive a speech or similar continuous information executes switching involving momentary interruption of information. Let this handover be referred to as hard handover, as distinguished from soft handover free from the above occurrence.
The probability that the above condition for soft handover cannot be satisfied increases when the distances from the MCC to the base stations are not the same. In light of this, while the MCC reports the head of a transmission unit of a radio interface to each base station, each base station inserts a preselected delay for a buffering purpose. Although this kind of scheme eases the condition for soft handover, it cannot surely guarantee soft handover. Further, because system synchronization is not achievable unless each mobile station is equipped with a GPS receiver, the inexpensive configuration of a terminal is limited.
With CDMA communication, it is possible to reduce transmission power by using cell diversity available with soft handover, to increase the number of mobile stations connectable to a single base station, and to thereby enhance the communication efficiency of the entire system, as stated earlier. However, in the systems taught in Documents 1 and 2, each base station is provided with a nondirectional antenna. Therefore, on an up-going link from a certain mobile station to a base station, communication quality is deteriorated due to interference caused by a signal sent from another mobile station. As a result, the number of mobile stations connectable to a single base station is reduced.
In order to solve the above problem, applying the TDMA digital cellular telephone technology disclosed in Document 3 to the CDMA communication system is now in study. The technology of Document 3 is such that each cell is subdivided into a plurality of sectors in order to enhance the efficient reuse of frequency, thereby increasing the number of mobile stations connectable to a single base station. However, no specific system configurations or control methods have been reported yet.